In comparison with a light source in which the light is generated by an incandescent body, for example, a thin wire filament, an LED has several advantages, such as better efficiency, lower heat loss, higher mechanical stability, and longer service life. At the beginning of the technical development, however, LEDs had comparatively low light outputs. In particular, the development of powerful LEDs in the shortwave visible spectral range of light proved problematic. Consequently, the generation of comparatively powerful white light by additive color mixing of the light of at least three LEDs that have a red, a green, and a blue emission wavelength or by an adequately broad visible LED emission spectrum of a single LED was not possible. For these reasons, for a long time, LEDs did not come into use for illumination purposes in which white light is necessary.
Now, it is possible to produce LEDs with emission wavelengths in all ranges of the visible light spectrum with light outputs that satisfy the requirements of a large number of conventional incandescent illumination devices. The use of an LED illumination device is thus often only an issue of price.
From DE 3,917,101 A1, a conventional incandescent illumination system is known, which enables the mixing of various hues and intensities of the emitted light.
WO99/31560 discloses a large number of devices and methods, in which in each case LEDs are used as light sources. LED illumination devices are disclosed, for example, in WO 97/48134, WO 98/40665, and WO 99/30537. In particular, in WO 98/40665 and WO 99/30537, devices are described in which the light is generated by a large number of LED light pixels arranged on a flat support. Each light pixel includes three LEDs: one LED with an emission wavelength in the red spectral range, one in the green, and one in the blue. By means of the controllable additive color mixing of the red, green, and blue LED light, the LED illumination device can emit light of any color temperature desired.
However, due to degeneration of the LED material, in particular of the light emitting layers, the intensity and wavelength of the LED light emitted changes over time. Despite LED currents kept constant, this results in the fact that the light of the LED illumination device also has altered intensity and color temperature. The known LED illumination devices have the disadvantage that such a change in color temperature and intensity of the light of the LED illumination device has to be accepted. However, precisely in professional sectors of use, the dependability that the illumination device emits light of the desired color temperature and intensity is of critical significance.
Consequently, the object of the invention is to provide an illumination device with light emitting diodes (LEDs) that can emit light of a large number of pre-set color temperature and intensity parameters, whereby the color temperature and the intensity parameters of the light emitted correspond to the pre-set color temperature and intensity parameters, with high dependability during the entire service life of the device.